This invention relates to an amorphous, magnetically soft body of rare-earth and transition metals (RE-TM) wherein a well-defined portion of the body has been tempered to obtain hard magnetic characteristics. The invention also relates to a method of making RE-TM bodies with regions of both soft and hard magnetism.
Many rare-earth transition metal compositions have superior magnetic properties. RE-TM magnet alloys contain one or more of the rare-earth metals combined in suitable proportions with one or more of the magnetic transition metals, such as iron, nickel, or cobalt. Permanently magnetized RE-TM materials may have two-to-six times the magnetic energy of aluminum-nickel-cobalt type (Alnico) magnets, and be fifty-times as resistant to demagnetization by an external magnetic field. Because the RE-TM alloys may have high magnetic coercivities and energies, even relatively small permanently magnetized bodies may serve in heavy-duty purposes such as stator magnets for DC motors.
Magnetic materials are generally classified as "hard" or "soft". A hard magnetic material is difficult to magnetize and to demagnetize. A body of hard magnetic material with residual magnetism is referred to as a "permanent" magnet. A soft magnetic material is easily magnetized and demagnetized. Coercivity is a measure of magnetic hardness. A soft magnet has a coercivity close to zero. A hard RE-TM alloy magnet typically has a coercivity of thousands of Oersteds.
It is known that certain RE-TM alloys can be transformed from a magnetically soft state to a magnetically hard state by a suitable heat treatment. For example, A. C. Clarke reported in Applied Physics Letters, Vol. 23, page 642 (1973) that a magnetically soft body of TbFe.sub.2 which has a substantially noncrystalline (amorphous) microstructure can be transformed into a hard magnetic state by heating to a suitable temperature above about 350.degree. C. The treated material is substantially crystalline in nature.
Herein, the process of heating an amorphous rare-earth transition metal alloy of low magnetic coercivity to a temperature below its melting temperature such that it is at least partially crystallized to a state where the alloy has relatively high magnetic coercivity is referred to as "tempering".
Table I lists a number of rare-earth transition metal alloys that can be sputtered onto a backing in an amorphous state in which they are magnetically soft. This list is not inclusive of all such alloys, but merely representative of RE-TM alloys believed to be adaptable for the practice of the invention.
TABLE I ______________________________________ SmCo.sub.5 Sm.sub.2 (Co.sub.0.5 Fe.sub.0.5).sub.17 TmFe.sub.2 Sm.sub.0.5 Pr.sub.0.5 Co.sub.5 SmFeCo ErFe.sub.2 PrCo.sub.3 SmFe.sub.2 DyFe.sub.2 Pr(Co.sub.0.9 Fe.sub.0.1).sub.5 PrFe.sub.2 HoFe.sub.2 YCo.sub.5 YFe.sub.2 TbFe.sub.2 Sm.sub.2 Co.sub.17 GdFe.sub.2 TbFe.sub.3 Sm.sub.2 (Co.sub.0.7 Fe.sub.0.3).sub.17 YbFe.sub.2 Tb.sub.2 Fe.sub.17 ______________________________________
There are applications wherein it would be desirable to induce magnetic coercivity in only a selected region of an amorphous rare-earth transition metal alloy of low magnetic coercivity. For example, a cylindrical stator magnet for a DC motor could be tempered in selected regions to optimize the magnetic field for motor operation. Before this invention, it was not known how to tailor the magnetics of a unitary RE-TM body to achieve well-defined regions of both hard and soft magnetism.